Superheater for a vapor generator



965 o. OLSEN ETAL 3,168,076

SUPERHEATER FOR A VAPOR GENERATOR Filed May 19, 1958 5 Sheets-Sheet 1 INVENTORS Oscar Olsen BY Raymond J. Boyer ATTORNEY Feb. 2, 1965 o. OLSEN ETAL SUPERHEATER FOR A VAPOR GENERATOR Filed May 19, 1958 5 Sheets-Sheet 2 INVENTORS Oscar Olsen BY Raymond J. Boyer AT TORN EY Feb. 2, 1965 o. OLSEN ETAL SUPERHEATER FOR A VAPOR GENERATOR 5 Sheets-Shet 3 Filed May 19, 1958 FIG.5

F N i WHIIHIHIIIIHIZIIIII FIG.6

INVENTORS Oscar Olsen Raymond J. Boyer ATTORNEY Feb. 2, 1965 o. OLSEN ETAL 3,168,075

SUPERHEATER FOR A VAPOR GENERATOR Filed May 19, 1958 5 Sheets-Sheet 4 INVENTORS 23a Oscar Olsen BY Raymond J. Boyer ATTORNEY Feb. 2, 1965 o. OLSEN ETAL 3,

SUPERHEATER FOR A VAPOR GENERATOR Filed May- 19, 1958 5 Sheets-Sheet 5 FIG.10

22c no.9

22C 23B /\g a? JNVENTORS Oscar Olsen BY Raymond J. Boyer flan.-

ATTORNEY United States Patent SUPERHEATER FOR A VAPGR GENERATUIR Oscar @isen, Albertson, N.Y., and Raymond 3. Buyer,

Westfield, NJL, assiguors to The Babcoclr 8; Wilcox Company, New York, N.Y., a corporation of New Jerse y Filed May 19, 1958, Ser. No. 736,109

3 Claims. (Cl. 122-510) This invention relates to a vapor generating and heating apparatus and more particularly the invention is concerned with a novel arrangement, construction and support of a vapor heater and its relationship to the other component parts of a vapor generator for producing superheated vapor at substantially constant high temperature over a wide range of vapor outputs.

In vapor or steam generators a somewhat fundamental type of superheater is a convection unit, for gas temperatures where the heat transfer by radiation is very small. In such superheaters the superheat curve for any given unit increases with boiler output because the efficiency of the entire unit decreases with output, that is, gas temperatures increase with output and at an increasing rate. Therefore, since convection heat transfer rates are almost a direct function of output, the total absorption per pound of steam increases in this type of super-heater.

On the other hand a radiant superheater receives its heat through radiation and practically none from convection. Because the furnace temperature does not increase in direct proportion to boiler output but at a considerable lesser rate, the curve of radiant superheat slopes downward with the boiler output. Thus, it had been heretofore customary to coordinate the characteristics of the opposite sloping superheat curves by connecting a radiant superheater and a convection superheater in series to attain a substantially flat superheat curve over a load range. This has been customarily accomplished by disposing a superheater section close to the high temperature gases generated in the furnace to receive its heat through radiation and another superheater section located in a gas pass remotely disposed from the furnace chamber for receiving its heat primarily through convection. Because the convection superheater in arrangements of this type is exposed to relatively cooler gases in the gas pass, a disportionate large amount of convection superheater surface is required as compared to radiant superheater surface requirements to attain the same end result under similar operating conditions.

It is an object of this invention to effectively obtain in a vapor generator relatively constant vapor superheat temperature by locating a combination radiant-convection, drainable type vapor heater intermediate the height of a furnace having an unobstructed gas flow therethrough to effect a reduction in the overall amount of vapor heating surface from that which is generally required in units utilizing conventionally arranged convection and radiant superheaters to satisfy a given set of pressures, temperatures, and vapor flow conditions.

As the vapor heater in accordance with this invenL on is disposed in the furnace and exposed to high temperature gases the known gas exposed means and manner of supporting such vapor heaters are not well suited in the instant invention. This is because the exposure of such supports to the high temperature gases would have an adverse effect on the same due to oxidation and corrosion.

In oil fired units the deleterious effects of high temperature on such supports are further aggravated due to excessive corrosion caused by the impurities, particularly vanadium pentoxide present in the fuel when the support metal temperatures exceed 1100 F.

it is another object of this invention to provide a novel support arrangement for a vapor heater disposed in a relatively high temperature Zone, wherein the supporting means therefor are disposed outside of the main gas flow path and hence out of direct contact with the high temperature heating gases.

The foregoing objects, related features and advantages are attained in a vapor generating and heating unit comprising essentially of a vertically elongated furnace defined by walls of generating tubes. Adjacent the upper end of the furnace and connecting with a furnace exit, is a gas pass defined by means including an upper liquid-vapor separating drum arranged to receive the liquid-vapor mixture from the wall tubes and a lower water drum connected to the upper drum by a plurality of generating tubes disposed in the gas pass. According to this invention vapor heating means, consisting of drainable primary and secondary heating sections of horizontally extending tubular elements, are disposed intermediate the height of the furnace and extend transversely thereof in a zone lying between the burner means and furnace gas exit, the vapor heating means being directly exposed to the flow of gases passing upwardly through the furnace. With this arrangement the absorption characteristics of each section of the vapor heater is such that the oppositely sloping superheater curves symbolic of a convection and radiation types of superheater are co-ordinated with an optimum amount of absorbing surfaces to produce substantially constant high superheat temperature throughout a vapor loading range.

The vapor heater is supported intermediate the height of the furnace in accordance with this invention by extending the end portions of the tubular vapor heater elements through and beyond the plane of opposed fluid cooled furnace walls, with the extending portion of at least the lowermost tubular elements supported by means secured to the circumferential portions of furnace wall tubes which are out of direct contact of the furnace gases. In this manner the support means are effectively protected from the harmful effects of erosion and oxidation in the case of coal firing and/or from the corrosive action due to oil impurities in oil firing.

A feature of this invention resides in the provision that the unit arrangement is such that it has a relatively low draft loss.

Another feature resides in the provision of an all drainable, combination, radiation-convection superheater wherein the vapor flow is in cross-flow heat transfer rela tionship with respect to the flow of heating gases.

Another feature of this invention resides in the provision that while the vapor heater is disposed in a high temperature zone of a furnace chamber, the means by which the vapor heater is supported therein are not exposed to the high temperature gases.

Still another feature resides in a novel support weldment or lug connected to a tubular element of the vapor heater for supporting the same.

Other features and advantages may be readily apparent when considered in view of the drawings and description in which:

FIG. 1 is a sectional vertical side view of the vapor generator in accordance with this invention.

FIG. 2 is an enlarged arrangement of the vapor heating section shown in FIG. 1 and support means therefor.

FIG. 3 is a section taken along line 3-3 of FIG. 2.

FIG. 4 is an enlarged detail front view of a typical su perheater support assembly in accordance with this invention, and having portions thereof broken away.

FIG. 5 is a side view of FIG. 4 with portions thereof broken away.

FIG. 6 is a plan view of FIG. 4 with portions broken away.

FIG. 7 is a partial view of the vapor heating unit and support tubes therefor taken along line 77 of FIG. 2.

PEG. 8 is a partial view taken along line 8-8 of FIG. 2.

FIG. 9 is a sectional plan taken along line 9'-9 of FIG. 2. I

FIG. 10 is a sectional plan view taken along line lit-14) of FIG. 2.

Referring to FIG. 1, there is shown in accordance with 23 and 24 respectively. Adjacent the upper end of the i furnace chamber 21 in the front wall is a furnace gas exit 25 communicating with a laterally extending gas pass 26. Extending transversely of the gas pass and forming 'a portion of the roof thereof is a steam and water separating drum 27. Disposed below the steam and water separating drum 27 and forming a portion of the floor of gas pass 26 is a lower water drum 28, the ends of which are extended slightly beyond the plane of the side wall tubes 24. 7

4 bule 23C in the vicinity of the primary and secondary superheater section 46 and 50 respectively. Intermediate of the superheater sections other tubes 23B forming another portion of the rear wall are .laterally displaced to complement the vestibule-forming tubes 23A in the vicinity of the primary superheater. Thus the displacement of tubes 23A of the rear wall results in a space S being formed between rear wall tubes 23 and 23B for accommodating the end portions of the secondary superheater tube 50 and the displacement of tubes 23B complementing tubes 23A in the vicinity of the primary superheater forms a double space S' for accommodating the ends of the primary superheater tubes, as will be hereinafter described.

A plurality of burners adapted to 'fire either coal, oil and/or gas are disposed adjacent the bottom of the furnace and are arranged to discharge hot products of combustion through burner ports in the front wall. The gases-generated flow vertically up through the furnace and thence laterally through the furnace exit 25 into the gas pass 26, the gases exiting from the gass pass through a duct 41 which directs the gases to a suitable heat trap 142, as shown. As the gases flow upwardly through the furnace only a circumferential portion of the front, side and rear wall tubes 22, 23 and 24 are in direct contact with the main stream of heating gases and absorbs the heat thereof by radiation, and the water rising therein being converted to a steam-water mixture. The resulting mixture is then ultimately discharged into the steam and water drum 27 and separated therein.

The steam thus separated is supplied through steam pipes 43 to a steam or vapor heater 44. In accordance with this invention the steam is first introduced into the inlet header and then enters the primary superheater A generating tube bank 29 disposed in the gas pass 26 interconnects the upper drum 27 with the lower drum 28. In operation, natural circulation between drums 27,

28 is established by a portion 29A of the generating tube bank 29 serving as downcomers to supply the lower drum 23 and another portion 21B of the tubes 29 dsposed upstream and in a relatively hotter gas zone serving as risers. The downcomers 29A also supply the downcomers 39 which are connected to each end of drum 28 and supply water to a lower rear wall header 31, which in turn sup- 7 defining the side wall extend betweentheir respective v lower and upper side wall headers 34 and 36. The upper header 36 in each side wall in turn is connected by one or more conduits 37 to the steam drum 27. Extending upwardly from the lower rear wall header 31 is a row of tubes which extend along the iloor 38 of the furnace as floor tubes 22A and then continue upwardly along the front wall of the furnace as tubes 22. A portion 22B of the front wall tubes 22 extend throughout the height of the front wall and connect directly to the lower drum. 28, discharging thereinto a steam and water mixture which is collected under a bafile 28A. T he steam and water mixture collected under baflle 28A is discharged therefrom through a row of tubes 28B and directed to drum 27.

Intermediate the height of the front wall portions 22C and 22B of tubes 22 are inwardly bent to form an arch "or nose 39, thence continue upwardly to connect into drum 27 with tube 22C forming the support tubes for the front end of the superheater. Beyond the superheater tube portions 220, 22E define a screen 22D in the vicinity of the furnace gas exit 25.

Referring to FIGS. 1, 2, 8, 9 and 10 some tubes 23A of rear wall tubes are laterally displaced to form a vestisection 45. As shown the primary section 46 includes a plurality of vertically disposed return bend tubular platen elements 36A transversely spaced across the width of the furnace chamber 21. The extending end portionsof the tubular elements or platens project through spaces 8' formed between adjacent tubes of the front and rear wall tubes 22 and 23 respectively as will be herein described. The discharge end of the tubular elements 46A connect into a primary outlet header 47 which in turn is serially connected by suitable piping 48 to the inlet header 49 of asecondary superheater section 50. 7

As shown, the secondary supenheater section 50 com- .prises a plurality of nested returnbend tubular elements 50A, 53B vertically disposed to form platens which are transversely spaced across the width of the furnace, the discharge end of the elements 50A, 50B connecting to the outlet header 51 from which the steam is delivered to a point of use, through suitable piping not shown.

In order that substantially constant superheat temperatures may be maintained over a load range, both the primary and secondary superheater section 46, 50 respec- ..t1vely, in accordance with this invention, are disposed in radiation cavity 2 1A. Accordingly the upper portion of the primary superheater section 46 which borders the upper radiation cavity 21A is predominantly heated by radiation from the. furnace gases in cavity 21A while the lower portion of the primary superheater section 46 rec'eives its heat primarily by convection. Likewise in the secondary superheater section 50 the lower portion thereof which borders on the lower radiation cavity 21B is predominantly heated by radiation, while the upper portion of the secondary superheater section 50 receives its heat primarily by convection, Consequently, with this arrangement both sections 46, 50 of superheater 44 in effect approach an ideal combination radiation convection type superheater in which the characteristic curves of a predominantly radiant and a predominantly convection superheater tare co-ordinated to produce a relatively flat superheat curve for the steam flowing therethrough. It thus follows that with this arrangement substantially constant superheat temperature can be maintained throughout a wide load range with a minimum amount of extraneous supcrheat control equipment, such as a Spray attemperator or the like, which would otherwise be necessary. Also, by utilizing a combination radiation-convection superheater section in the arrangement described, the total amount of heat absorbing surface required is substantially less than that wherein a predominantly convection type superheater is utilized in conjunction with a wholly radiant superheat for attaining comparable results. In accordance with this invention the amount of superheating surface is further maintained to an optimum minimum by providing the steam flow therethrough to be in cross-flow heat transfer relationship to the flow of heating gases passing thereover, the steam flowing through the primary superheater section 46 being in counter-flow relationship with respect to the gas flow and steam flowing in the secondary superheater section 50 being in parallel flow relationship with respect to the gases passing thereover.

Inasmuch as the superheater sections 46 and 50 are disposed in a relatively high temperature zone within the furnace, it is another object of this invention to provide a novel support arrangement for the particular superheater 44 described. Since the life of superheater supports is dependent on the type of material form which they are made, the gas temperatures to which they are subjected, and the corrosive, erosive nature of the residual ash resulting from the combustion of the fuel, it has heretofore been necessary to fabricate these supports from relatively expensive, high grade, heat resisting alloy such as a 2520 CrNi alloy in order to obtain reasonable service life from the supports. However, even when utilizing such high priced, heat resisting, alloy metals, the life of such supports has been materially shortened when they are exposed to high temperature heating gases such as those normally produced in a furnace. For example, it has been found that if the temperature of the supports exceeds, say 1150 F., vanadium compounds which may be present in the ash residue resulting from the combustion of fuel oil and/or sulphur compounds occurring in the residual ash resulting from the combustion of fuels containing sulphur, will produce an accelerated corrosion mechanism which quickly causes structural failure of the supports.

It is contemplated to avoid the foregoing difliculties occasioned by elevated temperatures of the supports by providing the superheater herein described with a support arrangement such that the supporting means will be disposed out of direct contact with the main body of heating gases flowing upward through the furnace.

Accordingly, as shown in FIGS. 1, 2 and 7 to 10, the tubular elements of the primary and secondary superheater 46, 50 extend transversely of the furnace chamber 21 and are arranged so that the end portions including the return bends, extend through the spaces S and S formed by the displacement of portions of the front and rear wall tubes 22 and 23, respectively. In the illustrated form, as shown in FIGS. 7 to 9, the secondary superheater elements or platens 50 are horizontally spaced on relatively wider centers than the platen elements 46 of the primary superheater. For this reason tubes 23B of rear Wall tubes are displaced outwardly at a point intermediate the primary and secondary superheater sections 46, 50 respectively to enlarge the spacing between support tubes 23 for accommodating the end portions of the more closely spaced primary superheater elements 46.

In the front wall tubes 22B are laterally disposed in the vicinity of the primary section 46 for the same reason. Depending on the tube spacing and tube sizes of the primary superheater elements 46, it may be necessary to offset the end portion 46 of some of the primary superheater elements 46 as shown in FIG. 10 to accommodate the same in the spaces S formed by the displacement of tube portions 22B and 23B of the front and rear tubes respectively in the vicinity of the primary superheater.

Referring to FIGS. 1, 2, 7 and 8 it will be noted that according to this invention at least the lowermost tube element in each of the platens of the primary and secondary sections 46, 50 is supported by means 60, 60A disposed outside of the main gas flow path which is defined by the furnace tubes 22, 22C in the front, wall, tubes 23, 23A and 23B in the rear wall and tubes 24 in the side walls.

Referring to FIGS. 4, 5 and 6 the support means 60 and 60A each comprises lugs 61 welded to a circumferential portion of the front and rear wall support tubes and positioned so as to be shielded from the stream of heating gases flowing upwardly through the furnace chamber, i.e. located on the back side of its respective support tube, a bridge 62 supported on at least two lugs 61 and an improved weldment 70 connected to the superheater element or tube 46A and 50A for supporting the same on the bridge 62. Each lug 61 welded to the wall tubes consists essentially of an L shaped member having perpendicularly disposed leg portions 61A and 61B; the horizontal leg 61A portion having an arcuate edge portion 61C engaging the outside circumference of the wall support tubes 23. The vertical leg portion 61B is spaced from the support tube 23. Intermediate the width of the lug 61 is a gusset or web portion 61D extending above and below the horizontal leg portion 61A, the vertical straight edge 61E of the web 611) arranged to engage the support tube is secured thereto by welding. Thus the gusset or web 61D enables the lug 61 to have a sufficient portion thereof in engagement with the support tubes to insure a welded connection strong enough to support the superheater.

A bridge member 62 supported on the lugs 61 spans the space between adjacent wall tubes 23. As shown in FIGS. 4 and 5 the bridge member 62 comprises a casting which is shaped so that it may be engaged by at least two support lugs 61. Accordingly, the bridge casting 62 includes at least a pair of vertically disposed channel shaped portions 63, each arranged to form a housing 64 for receiving a lug 61. To insure stability of the bridge member 62 the edges of the opposed leg portions 63A of each channel are arranged to engage the sides of a support tube. The opposed leg portion 63A of each channel portion 63 has inwardly projecting bosses 65 which are adapted to straddle the web or gusset 61D of the lug 61 and rest upon the horizontal leg 61A of the lug. A top portion 66 connects the two channel portions 63 to form the bridge or span between support tubes; the outer edge of the top piece 66 having an upturned flange portion 67. If desired a gusset plate 68 may be secured between the inner leg portion 63A of each channel portion 63 to reinforce the bridge. In FIGS. 7 and 8, the spacing between support tubes, e.g. between tubes 23 in the rear wall and tubes 22C in front furnace wall, may necessitate the bridge to be modified by providing outer wing portions 66A at the ends of the top piece 66 as shown. This modification, however, does not in any way change the lug 61 or its function.

Connected to the end portion of the superheater tube or element projecting beyond the plane of the wall tubes for supporting the same on the bridge is a weldment or lug 76. In accordance with this invention the weldment 76 includes an arcuate cradle portion 71 with a depending stem portion 72 connected thereto. Referring to FIG. 4, it is to be noted that the arcuate inner surface 71A of the cradle '71 is of a radius which is less than that of the superheater tube 50A supported thereon. With the cradle construction of lug 70 described a two point line contact support for superheat'er tube 50A is insured even though the superheat'er tube may be out of round at this end thereof due to the formation of the return bonds. This feature is important in that it facilitates an adequate Weld for securing lug '70 to tube 50A. Further the space 73 formed between the outside circumferential surface of the tube and the inner surface 71A of the lug cradle tends to increase the temperature gradient or drop between the tube 50A and lug 70, and thereby tends to reduce the thermal stresses in lug 70. Also the line contact between the tube 50A and the cradle 71 of the lug at only two points on the tube circumference enables the entire circumferential wall of the tube 50A to be at substantially uniform temperature, thereby minimizing the stresses withinthe wall of the superheater tube.

From the foregoing description of the support means 60 and 60A it will be noted that the bridge casting 62 is loosely supported on support lugs 61 and is free to move relative thereto. Also the superheater'tube weldment 70 rests freely on the bridge casting 62 and is free to move relative thereto during thermal expansion of the parts, the depending projection 74 of the Weldment 70 co-operating with the flange 6'7 to limit the amount of relative expansion betweenthe bridge 62 and weldment 70.

As shown in FIGS. 2 and 3 the superposed tube lengths of the superheater platens are supported on saddles 75,

the arcuate portion 75A of the saddle being formed tov circumfercntially embrace the tube 503 supported'thereon with the spaced depending leg portion 753 secured to a subjacent tube portion as for example 50A disposed immediately below the supported tube 5GB.

From the foregoingdescription it is to be noted that the instant arrangement is conducive to very low draft loss, since by disposing the superheater sections 46, 56 below the furnace gas exit 25, the latter remain unobstructed and the gases are free to flow out of the furnace with a minimum of interference. Further the particular arrangement with respect to the other components of the steam generator enables superheated steam to be maintined at substantially constant temperatures throughout a wide load range with a minimum of temperature control equipment with an optimum amount of total heat absorb- Also the superheater support arrangement ing surfaces. is such that the useful life of the support means are greatly increased while at the same time enabling them to be fabricated from a lower quality of alloy metal which results in reducing the cost thereof proportionately. For

reference .to a particular embodiment thereof, it is to be appreciated that the invention is not to be taken as limited to all of the details thereofas' modifications and variations thereof may be made without departing from the spirit or scope of the invention.

What is claimed is:

1.- In a vapor generator having a fluid circulation 'system and an upright furnace chamber having a heating gas outlet at one end and supplied at its opposite end with high temperature heating gases and formed by Walls including radiant heat absorbing vapor generating tubes connected into said fluid circulation system, adrainable superheater disposed across the flow of gases in said furnace at a location between the point of gas supply and the gas outlet and connected for series vapor flow from said radiant heat absorbing vapor generating tubes and subdividing said furnace into upper and lower substantlally unobstructed radiation chambers, said superheater constituting substantially the entire vapor superheating surface of said generator and comprising a plurality of horizontally spaced tube platens arranged in vertical planes and extending across substantially the full width of the furnace, each platen including a multiplicity of horizontally extending return bend tubes, said superheatcr having its heating surface proportioned and arranged so that the superheater' tube portions adjacent the upper and lower radiation chambers absorb heat mostly by direct radiation and the remaining intervening superheater tube portions absorb heat chiefly by convection to provide a substantially constant vapor temperature leaving the superheater over a wide range of loads.

2. In a'vapor generator'having a fluid circulation system and an upright furnace chamber having a heating gas outlet at one end and supplied at its opposite end with high temperature heating gases and formed by walls including radiant heat absorbing vapor generating tubes connected into said fluid circulation system, vapor heating surface consisting of a vapor heater disposed across the flow of gases in said furnace at a location between the point of gas supply and the gas outlet and connected for series vapor flow from said radiant heat absorbing vapor generating tubes and sub-dividing said furnace into upper and lower substantially unobstructed radiation chambers, said vapor heater constituting substantially the entire vapor heating surface'of said generator and comprising a plurality of'horizontally spaced tube platens arranged in vertical planes, each platen including a multiplicity of horizontally arranged return bend tubes extending across the full width of said furnace, said vapor heater having its heating surface proportioned and arranged so that the upper and lower portions of said vapor istics of vapor flowing through the vapor heater to obtain finady superheated vapor at substantially constant temperature over a widely varying load range of the vapor generator.

3. In an upright furnace chamber having a heating gas outlet at one end and supplied at its opposite end with high temperature heating gases and formed by Walls including radiant heat absorbing vapor generating tubes, said walls including a pair of oppositely disposed walls,

'a drainable superheater disposed across the flow of gases in said furnace at a location between the point of gas supply and the gas outlet and connected for series vapor flow from said radiant heat absorbing vapor generating tubes and sub-dividing said furnace into upper and lower substantially unobstructed radiation chambers, said superheater comprising a plurality of horizontally spaced tube platens arranged in vertical planes, and extending across substantially the full Width of the furnace, each platen including a multiplicity of vertically spaced horizontal tubes extending across the full width of said furnace and through and outwardly beyond said pair of walls of said furnace, said superheater having its heating surface proportioned and arranged so that the superheater tube portions adjacent the upper and lower radiation chambers absorb heat mostly by direct radiation and the remaining intervening superheater tube portions absorb heat chiefly by convection to provide asubstantially constant vapor temperature leaving the superhcater over a wide range of loads, and meansfor supporting the weight of said superheater on the vapor generating tubes of said pair of walls at a location out of contact with the heating gases flowing through said furnace, while permitting longitudinal movement of the superheater tubes and restraining lateral and vertical movements thereof.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Jacobus et al. Sept. 24, 1935 Gilg Nov. 1, 1938 Brown Oct. 10, 1939 Armacost July 10, 1951 Rowand et a1. Mar. 13, 1956 Marshall Aug. 20, 1957 Koch et a1. Nov. 5, 1957 10 Nunninghofi July 29, 1958 Heller Oct. 21, 1958 Hochmuth Nov. 15, 1960 FOREIGN PATENTS Great Britain May 19, 1954 Great Britain Apr. 9, 1958 France July 4, 1938 France Apr. 8, 1953 

1. IN A VAPOR GENERATOR HAVING A FLUID CIRCULATION SYSTEM AND AN UPRIGHT FURNACE CHAMBER HAVING A HEATING GAS OUTLET AT ONE END AND SUPPLIED AT ITS OPPOSITE END WITH HIGH TEMPERATURE HEATING GASES AND FORMED BY WALLS INCLUDING RADIANT HEAT ABSORBING VAPOR GENERATING TUBES CONNECTED INTO SAID FLUID CIRCULATION SHSTEM, A DRAINABLE SUPERHEATER DISPOSED ACROSS THE FLOW OF GASES IN SAID FURNACE AT A LOCATION BETWEEN THE POINT OF GAS SUPPLY AND THE GAS OUTLET AND CONNECTED FOR SERIES VAPOR FLOW FROM SAID RADIANT HEAT ABSORBING VAPOR GENERATING TUBES AND SUBDIVIDING SAID FURNACE INTO UPPER AND LOWER SUBSTANTIALLY UNOBSTRUCTED RADIATION CHAMBERS, SAID SUPERHEATER CONSTITUTING SUBSTANTIALLY THE ENTIRE VAPOR SUPERHEATING SURFACE OF SAID GENERATOR AND COMPRISING A PLURALITY OF HORIZONTALLY SPACED TUBE PLATENS ARRANGED IN VERTICAL PLANES AND EXTENDING ACROSS SUBSTANTIALLY THE FULL WIDTH OF THE FURNACE, EACH PLATEN INCLUDING A MULTIPLICITY OF HORIZONTALLY EXTENDING RETURN BEND TUBES, SAID SUPERHEATER HAVING ITS HEATING SURFACE PROPORTIONED AND ARRANGED SO THAT THE SUPERHEATER TUBE PORTIONS ADJACENT THE UPPER AND LOWER RADIATION CHAMBERS ABSORB HEAT MOSTLY BY DIRECT RADIATION AND THE REMAINING INTERVENING SUPERHEATER TUBE PORTIONS ABSORB HEAT CHIEFLY BY CONVECTION TO PROVIDE A SUBSTANTIALLY CONSTANT VAPOR TEMPERATURE LEAVING THE SUPERHEATER OVER A WIDE RANGE OF LOADS. 